Systems and methods for optically generated trigger multiplication

ABSTRACT

Systems and methods for providing trigger signals in an optical interrogator, wherein multiple triggers are generated within each period of a varying reference signal, and wherein the triggers are evenly spaced according to the wavenumber of the reference signal. In one embodiment, an optical frequency domain reflectometry system provides a laser beam to a reference interferometer to produce a reference signal. This signal is passed through a 4×4 optical coupler which splits the signal into a first signal and a second signal that is 90 degrees out of phase with the first signal. These signals are converted to electrical signals, and a trigger unit generates triggers at points at which the two electrical signals have zero-crossings, and at which the magnitudes of the signals are equal. The resulting triggers remain evenly spaced within the period of the reference signals, even when the period is changed.

BACKGROUND

1. Field of the Invention

The invention relates generally to interferometry and more particularlyto systems and methods for generating multiple triggers for each cycleof a sampling signal in an optical interrogator.

2. Related Art

The commercial success of optical fiber telecommunications has fosteredthe growth of optical fiber sensing applications by providing a readysupply of low cost, high quality components and test equipment. Anotherenabling technology for fiber sensing was the discovery of theultraviolet (UV) photosensitivity in optical fiber. Photosensitivityallows the alteration of the internal structure of a fiber waveguide.Modification of the waveguide can be employed for a number of usefulpurposes, one of which is to induce a periodic modulation of therefractive index along the fiber core to create a wavelength selectivereflector called a fiber Bragg grating (FBG). FBG's can be producedconveniently and very inexpensively during the fiber draw process. Theperiod of the modulation of the refractive index determines thewavelength reflected by the FBG. After the FBG is formed, the gratingperiod of the FBG can be physically altered by changing the mechanicalload on the fiber, or by changing its temperature. By monitoring thewavelength reflected from a FBG, the FBG can be used as a transducer forboth strain and temperature.

An important recent development in the use of FBGs is the use of opticalfrequency domain reflectometry (OFDR), which is a sensing technique thatcan be used to monitor FBGs or other sensors. This technique can be usedto interrogate hundreds or thousands of FBG's distributed along thelength of a single optical fiber. The OFDR technique has manyapplications where light weight, immunity to electromagneticinterference, high sensor density, and remote readout are importantconsiderations. These applications include monitoring sensors such asFBGs, providing diagnostics on optical fiber networks and cables,including the intrinsic Rayleigh scatter of the optical fiber,monitoring the condition of aerospace structures, monitoring industrialprocesses, and monitoring sub-marine and oil well systems.

One of the problems with conventional OFDR systems of the type describedabove is that the sinusoidal reference signal provides sampling triggersonly once per period, for instance at rising zero-crossings. When areference interferometer is used for sampling, the frequency of thesampling limits the frequency of signals from the device under test thatcan be resolved by the system (because of the Nyquist criteria), whichin turn limits the length of the device under test. If it is desired toincrease the sampling frequency and thereby increase the possible lengthof the device under test, the path length difference of the referenceinterferometer must be increased. This may present practicaldifficulties, however. It would therefore be desirable to providesystems and methods for increasing the rate of the sampling signalwithout increasing the length of the reference interferometer.

SUMMARY OF THE INVENTION

This disclosure is directed to systems and methods for providing triggersignals in an optical interrogator that solve one or more of theproblems discussed above. In particular, the systems and methods providefor the generation of multiple triggers within each period of a varyingreference signal, where the triggers are substantially evenly spacedaccording to the wavenumber of the reference signal.

In one embodiment, an OFDR system is used to interrogate a sensor arraythat is embedded in an optical fiber. A laser illuminates both thesensor array and a reference interferometer. The referenceinterferometer produces a reference signal that is passed through a 4×4optical coupler which splits the signal and provides output signals thatinclude a first signal and a second signal that is 90 degrees out ofphase with the first signal. Optical detectors are used to convert theseoptical signals to electrical signals. A trigger unit then detectspoints at which the two electrical signals have zero-crossings, and atwhich the magnitudes of the signals are equal. This produces eighttriggers per period of the reference signals, rather than the singletrigger that is normally produced at the rising zero-crossing of thereference signal. Further, because the triggers are generated fromevents that correspond to fixed positions within the period of thereference signals, the triggers remain approximately evenly spacedwithin the period of the reference signals, even when the period changesas a result of non-linearities in the sweeping of the frequency of thelaser.

An apparatus for triggering uniform wavenumber sampling in an opticalfrequency domain reflectometer system. The apparatus includes an opticalcoupler, at least one reference optical detector, and a trigger unit.The optical coupler is configured to receive a reference optical signalfrom a reference interferometer and to provide at least one outputoptical signal having the same period as the reference optical signal.The reference optical detector is configured to receive the outputoptical signal and to convert this signal into at least one electricalsignal having the same period as the output optical signal. The triggerunit is configured to receive the electrical signals and to generate atrigger signal that contains more than one trigger per period of theelectrical signals, where the triggers have uniform wavenumber spacing.In one embodiment, the optical coupler is configured to split thereference optical signal into two optical signals that are 90 degreesout of phase with each other (as are the two resulting electricalsignals). The optical coupler may be, for example, a 4×4 opticalcoupler. The trigger unit may be configured to generate triggers atzero-crossings of the first and second electrical signals and at timesat which the first and second electrical signals have equal magnitudes.For instance, the trigger unit may include fourcomparator-differentiator pairs and a summing unit, where a firstcomparator-differentiator pair receives the first and second electricalsignals as inputs, a second comparator-differentiator pair receives thefirst electrical signal and an inverse of the second electrical signalas inputs, a third comparator-differentiator pair receives the firstelectrical signal and ground as inputs, and a fourthcomparator-differentiator pair receives the second electrical signal andground as inputs. The output of each comparator-differentiator pair canthen be provided as an input to the summing unit, the output of which isthe trigger signal. The apparatus may also include a referenceinterferometer and a laser, where the reference interferometer isconfigured to receive a laser light beam from the laser and to producethe reference optical signal.

Another embodiment comprises a method for triggering uniform wavenumbersampling in an optical frequency domain reflectometer system. The methodincludes providing a reference optical signal, converting the referenceoptical signal into at least one electrical signal having the sameperiod as the reference optical signal, and generating a trigger signalbased on the at least one electrical signal, where the trigger signalcontains more than one trigger per period of the at least one electricalsignal, and where the triggers have uniform wavenumber spacing. Thereference optical signal may be generated by providing a laser beam to areference interferometer, the output of which is passed through a 4×4optical coupler to produce a pair of optical signals that are 90 degreesout of phase with each other. These optical signals are then convertedby optical detectors to electrical signals that are 90 degrees out ofphase. The triggers may then be generated at zero-crossings of the twoelectrical signals and at times at which the two electrical signals haveequal magnitudes.

Another embodiment comprises an OFDR system that includes a laser and afirst optical coupler (e.g., a 4×4 coupler) that couples the laser to areference interferometer. The reference interferometer uses the laserlight beam to produce a reference optical signal, which is returned tothe first optical coupler. Reference optical detectors receive thereturned output optical signals (which are 90 out of phase with eachother) and convert them into corresponding electrical signals having thesame period as the output optical signals. A trigger unit receives theelectrical signals and generates a trigger signal that contains morethan one trigger per period of the at least one electrical signal. Thetriggers have uniform wavenumber spacing. For instance, the trigger unitmay use comparator-differentiator pairs to generate triggerscorresponding to zero-crossings of the first and second electricalsignals and times at which the first and second electrical signals haveequal magnitudes. The system also includes a sensor array that receivesthe beam from the laser and produces an optical sensor signal. A sensorarray optical detector receives the optical sensor signal and thetrigger signal. The sensor array optical detector samples the opticalsensor signal in response to occurrences of the triggers in the triggersignal. The sensor array may be a sensor array interferometer. Thesensor array interferometer may, for example, include a 2×2 opticalcoupler and a plurality of selectively reflective sensors, such as fiberBragg gratings. A first port of the 2×2 optical coupler receives thelaser light beam, a second port of the 2×2 optical coupler is coupled toa first optical fiber that terminates at a broadband reflector, and athird port of the 2×2 optical coupler is coupled to a second opticalfiber that incorporates the sensors.

Numerous other embodiments are also possible.

BRIEF DESCRIPTION OF THE DRAWINGS

Other objects and advantages of the invention may become apparent uponreading the following detailed description and upon reference to theaccompanying drawings.

FIG. 1 is a diagram illustrating a basic OFDR system in accordance withthe prior art.

FIG. 2 is a diagram illustrating a reference optical signal and atrigger signal that includes triggers at rising zero-crossings of thereference optical signal.

FIG. 3 is a diagram illustrating an OFDR system that generates multipletriggers per period of a reference optical signal in accordance with oneembodiment.

FIG. 4 is a diagram illustrating an exemplary structure for a triggerunit as may be used in the system of FIG. 3.

FIG. 5 is a diagram illustrating a pair of reference optical signals anda trigger signal that includes multiple triggers per period of thereference optical signals as generated by the trigger unit of FIG. 4.

While the invention is subject to various modifications and alternativeforms, specific embodiments thereof are shown by way of example in thedrawings and the accompanying detailed description. It should beunderstood, however, that the drawings and detailed description are notintended to limit the invention to the particular embodiment which isdescribed. This disclosure is instead intended to cover allmodifications, equivalents and alternatives falling within the scope ofthe present invention as defined by the appended claims.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

One or more embodiments of the invention are described below. It shouldbe noted that these and any other embodiments described below areexemplary and are intended to be illustrative of the invention ratherthan limiting.

As described herein, various embodiments of the invention comprisesystems and methods for providing trigger signals in an opticalinterrogator, wherein multiple triggers are generated within each periodof a varying reference signal, and wherein the triggers are evenlyspaced according to the wavenumber of the reference signal.

In one embodiment, an OFDR system is used to interrogate a sensor arraythat is embedded in an optical fiber. A laser provides a beam to boththe sensor array and a reference interferometer. The referenceinterferometer produces a reference signal that is passed through a 4×4optical coupler which splits the signal and provides output signals thatare 90 degrees out of phase with each other. Optical detectors are usedto convert these optical signals to electrical signals. A trigger unitthen detects points at which the two electrical signals havezero-crossings, and at which the magnitudes of the signals are equal.This produces eight triggers per period of the reference signals, ratherthan the single trigger that is normally produced at the risingzero-crossing of the reference signal. Further, because the triggers aregenerated from events that correspond to fixed positions within theperiod of the reference signals, the triggers remain evenly spacedwithin the period of the reference signals, even when the period changesas a result of sweeping the frequency of the laser.

OFDR is a technique that can be used to monitor, or interrogate, sensorssuch as FBGs that have reflectivity which is wavelength-dependent. FBGsare devices that utilize periodic modulations of the refractive index ofan optical fiber to achieve wavelength-selective reflection. The periodof the modulation determines the wavelength that is reflected, but theperiod of the modulation (hence the wavelength reflected) can be alteredby changing the mechanical load on the fiber or the temperature of thefiber. The FBG can be used as a sensor for strain and/or temperature bymonitoring the wavelength that is reflected by the FBG.

Because of the availability of techniques such as OFDR, many FBGs (e.g.,hundreds or even thousands) can be distributed along the length of asingle fiber. In one configuration, OFDR system generates a samplingsignal, for example, by using a frequency-swept laser with an in-fiberreference interferometer. The light from the laser is also provided tothe fiber that incorporates the FBGs. Each of the FBGs effectivelyprovides an interferometer output which is sampled according to thesampling signal produced by the reference interferometer. The lightreflected by each FBG is modulated by a unique frequency that isdependent upon the FBG's location along the length of the fiber.

A basic, conventional OFDR system is shown in FIG. 1. This systemincludes a wavelength-tunable laser 110, three 2-by-2 optical couplers120-122, two photodiode detectors 130-131, an FBG-array-basedinterferometer 140, and an in-fiber interferometer 150. The laser lightis split by coupler 120 and travels to couplers 121 and 122. The port ofcoupler 120 that is not used is terminated (as noted in the figure by an“X”).

Each of the 2-by-2 optical couplers splits the light that passes throughthe coupler, so light entering one side of the coupler is split andoutput on the two ports on the opposite side. The coupler works the samein both directions. Coupler 121 is used to form an in-fiberinterferometer with the light reflected from reflectors 151 and 152.This light is detected by a detector 130, such a photodiode. Thisreference interferometer has an optical path length difference of 2nL,where n is the effective refractive index of the fiber and L is the pathdifference of the two paths through the interferometer (from coupler 121to reflector 151, and from coupler 121 to reflector 152).

Coupler 122 is used to form what is effectively a sequence of similaroverlapping interferometers. The first path of each interferometer isformed between coupler 122 and reflector 141. The second path of eachinterferometer is formed between coupler 122 and the respective one ofthe FBGs (e.g., 142, 143 or 144). The light reflected from reflector 141and each grating (142-144) is detected by detector 131.

The signal detected by detector 130 is sinusoidal. The phase of thissignal is a linear function of the wavenumber of the laser. The signaldetected by detector 130 is converted to an electrical signal by triggerunit 160. This signal is used to trigger sampling of the sensor signalsarriving at detector 131. Typically, the rising zero-crossings of thesinusoidal signal detected by detector 130 are used as triggers. This isshown in FIG. 2. The reference signal I detected by detector 130 isdepicted at the top of FIG. 2. The trigger signal T produced by triggerunit 160 is shown at the bottom of the figure. As indicated in thisfigure, rising zero-crossings 210 and 211 are detected by trigger unit160, which generates corresponding triggers 220 and 221 in the triggersignal.

The data detected by detector 131 is forwarded to a data processing unit170. Using the signal detected by detector 130 to trigger sampling ofthe optical signals arriving at detector 131 provides high resolution,and also provides uniform wavenumber sampling of these signals. Itshould be noted that, because the frequency of the laser is swept,time-synchronous sampling may not provide uniform sampling, if thetuning is non-linear with respect to time. The high-resolution, uniformsampling allows discrete Fourier analysis of the signals received atdetector 131.

The system of FIG. 1 can easily be extended to include additional sensormodules. Each additional sensor module may be constructed in the samemanner as the first, including a detector (e.g., 131) and asensor-array-based interferometer (e.g., 140). If optical coupler 120 ischanged, for example, to a 4×4 coupler, the light from laser 110 can beprovided to one or two additional sensor modules. The detector of eachadditional sensor module can be triggered using the same trigger signalgenerated by trigger unit 160, and the data can be provided to andprocessed by the same data processing unit

As noted above, the light reflected by each FBG is modulated by a uniquefrequency that is dependent upon the FBG's location along the length ofthe fiber. The farther an FBG is located from the coupler, the higherthe frequency associated with the FBG. In order to increase the lengthof the fiber (hence the distance of the FBGs from the coupler), thesystem of FIG. 1 must also increase the path length difference of thereference interferometer. Because it may not be practical or convenientto increase the length of the reference interferometer, it may bepreferable to maintain the length of the reference interferometer andmultiply the triggers that are generated based on the reference signaldetected by detector 130.

The OFDR signals are driven by the wavelength tuning of the laser. Asthe laser is tuned, the signal at detector 130 is given byD1=cos(k2nL)   (1)

The frequency of this signal is proportional to L, the interferometerpath length difference. The constant k is the wavenumber of the light,and is related to the light's wavelength, λ, byk=2π/λ  (2)

The interferometer cycles once for a wavenumber change, Δk, ofΔk=π/nλ  (3)

This is a constant. The interferometer therefore cycles linearly as afunction of wavenumber. The positive-going zero crossing of the signalat detectcor 130 are used to trigger the sampling of the signal atdetector 131. This guarantees that the signal at 131 is sampled at theconstant wavenumber interval given by Equation (3).

The signals corresponding to all of the FBGs are present at detector131. Each of these signals is similar to the signal at detector 130, burthe response of each FBG is limited to the narrow wavelength range, orspectrum, over which it reflects. In other words, the individualinterferometer corresponding to each FBG can only produce an outputsignal when it is reflecting light. The signal at detector 131 is thesum of these individual interferometer responses, so the signal atdetector 131 can be written as

$\begin{matrix}{{D\; 2} = {\sum\limits_{i}{R_{i}{\cos\left( {k\; 2{nL}_{i}} \right)}}}} & (4)\end{matrix}$

where Ri is the spectrum of the i'th grating and Li is the path lengthdifference of the corresponding i'th interferometer. This equationshows, as noted above, that the spectrum of each grating is modulated bya signal with a unique frequency which is governed by the grating'sposition, Li, in the fiber. By bandpass filtering around a specificfrequency (location) via fast Fourier transform, the spectrum of eachgrating can be independently measured and strain or temperatureinferred.

Referring to FIG. 3, an OFDR system that provides multiple triggers perperiod of the reference interferometer signal is shown. This systemincludes a wavelength-tunable laser 310, three optical couplers 320-322,three photodiode detectors 330-332, an FBG-array-based interferometer340, and an in-fiber interferometer 350.

As in the system of FIG. 1, the laser light is split by coupler 320 andtravels to couplers 321 and 322. Couplers 320 and 321 are 2×2 couplers.Coupler 322, however, is a 4×4 coupler, so light entering one side ofthis coupler is split and output on four ports on the opposite side. Twoof the ports on the right side of coupler 322 are connected to opticalfibers that terminate at broadband reflectors 351 and 352, forming areference interferometer which is essentially the same as the referenceinterferometer of FIG. 1. The reference interferometer of FIG. 3 has anoptical path length difference of 2nL, where n is the effectiverefractive index of the fiber and L is the path difference of the twopaths through the interferometer. In the system of FIG. 3, however, theinterferometer's output signal is returned to two detectors, 330 and331, as well as to port 325, which is unused. The output signal receivedby detector 330 is 90 degrees out of phase with the signal received bydetector 331. As in FIG. 1, coupler 321 is used to form effectivelyoverlapping interferometers 340. The light reflected from reflector 341and each FBG (342-344) is detected by detector 332.

The signals detected by detectors 330 and 331 are sinusoidal and havephases that are linear functions of the wavenumber of the laser. Asnoted above, the signals are 90 degrees out of phase. The opticalsignals detected by detectors 330 and 331 are converted to electricalsignals by trigger unit 360. Trigger unit 360 makes several comparisonsof these electrical signals with each other and with reference voltages,and generates trigger signals based on the comparisons. The triggersignals are used to trigger sampling of the optical signals arriving atdetector 332. The data detected by detector 332 is forwarded to a dataprocessing unit 370, which processes the data to determine thewavelength of the light reflected from each of the FBGs.

In one embodiment, trigger unit 360 is configured to generate eighttrigger pulses uniformly distributed throughout the period of thesinusoidal optical signals. An exemplary structure for the trigger unitis shown in FIG. 4. The trigger unit of FIG. 4 receives the electricalsignals, I and Q, which correspond to the optical signals received bydetectors 330 and 331, and generates a trigger signal, T, which is usedto trigger sampling at detector 332.

The trigger unit identifies zero-crossings (both rising and falling) ofsignals I and Q, and also identifies points at which the magnitudes of Iand Q are equal. The zero-crossings occur every 90 degrees during theperiod of the signals. Similarly, the magnitudes of the signals areequal every 90 degrees during the period of the signals, but these occurwith a 45 degree phase difference from the zero-crossings. Whencombined, these events occur every 45 degrees throughout the period ofthe signals, resulting in eight evenly spaced trigger events during theperiod of the signals.

Referring to FIG. 4, the trigger unit includes a set of comparators410-413, a set of differentiators 420-423, and a summing unit 430. Inthis embodiment, comparators 410-413 comprise Schmidt triggers withnegligible hysteresis. Comparators 410-411 and differentiators 420-421are used to produce triggers corresponding to points at which theoptical signals have equal magnitude, while comparators 412-413 anddifferentiators 422-423 are used to produce triggers corresponding tozero-crossings.

Comparator 410 receives signals I and Q as inputs. When I is greaterthan Q, the output of comparator 410 is high. When I is less than Q, theoutput of comparator 410 is low. Thus, the output of comparator 410transitions between high and low whenever I and Q are equal. The outputof comparator 410 is input to differentiator 420. Differentiator 420generates an output pulse whenever there is a transition in the inputsignal. Consequently, differentiator 420 produces an output pulsewhenever I and Q are equal. The output of differentiator 420 is thenprovided as an input to summing unit 430. Comparator 411 receives signalQ and the inverse of signal I as inputs. Comparator 411 couldalternatively receive I and the inverse of Q. When signals I and Q areequal in magnitude, but opposite in sign, the output of comparator 411transitions between high and low. The output of comparator 411 isprovided as an input to differentiator 421, which converts eachtransition of the input signal to a pulse at the output of thedifferentiator. The output the differentiator 421 is provided as aninput to summing unit 430.

Signal I is provided as an input to comparator 412. The other input ofcomparator 412 is tied to ground. When signal I is greater than zero,the output of comparator 412 is high, and when the signal is less thanzero, the output of the comparator is low. The output of comparator 412transitions between high and low whenever signal I crosses zero. Theoutput of comparator 412 is provided as an input to differentiator 422.Differentiator 422 produces a pulse at its output whenever there is atransition between high and low in the input signal. The output ofdifferentiator 422 is provided as an input to summing unit 430. Signal Qis provided as a first input to comparator 413. The second input ofcomparator 413 is tied to ground, so that the output of the comparatortransitions between high and low whenever signal Q crosses zero. Theoutput of comparator 413 is provided as an input to differentiator 423.Differentiator 423 produces an output pulse whenever there is a high-lowor low-high transition in the input signal, so in output pulse isgenerated for each zero-crossing of signal Q.

As noted above, the outputs of differentiators 420-423 are provided asinputs to summing unit 430. Each of these inputs is low, except when apulse is generated for the corresponding trigger conditions (i.e., I=Qfor differentiator 420, I=−Q for differentiator 421, I=0 fordifferentiator 422, and Q=0 for differentiator 423). Consequently, theoutput of summing unit 430 is low, except when a pulse is received fromone of differentiators 420-423.

Referring to FIG. 5, a diagram illustrating the relationship betweensignals I and Q and the output of the trigger unit is shown. In theupper part of the figure, signals I and Q are depicted. The output ofthe trigger unit is shown in the lower part of the figure. Signals I andQ are the electrical signals that are output by detectors 330 and 331.Signals I and Q track the optical signals that are produced by referenceinterferometer 350 and optical coupler 322—they have the same period,phase and waveform. When signals I and Q are input to the trigger unitshown in FIG. 4, the trigger unit produces a trigger signal, T, as shownat the bottom of FIG. 5. The pulses (triggers) in trigger signal Tcorrespond to the events detected by the comparator-differentiator pairsin the trigger unit. Comparator-differentiator pair 410/420 detects thepoints at which I and Q are equal (e.g., 513, 517) and generatescorresponding triggers (e.g., 523, 527). Comparator-differentiator pair411/421 detects the points at which I and Q are equal in magnitude, butopposite in sign (e.g., 511, 515) and generates corresponding triggers(e.g., 521, 525). Comparator-differentiator pair 412/422 detects thezero-crossings of signal I (e.g., 510, 514, 518) and generates triggersfor these events (e.g., 520, 524, 528). Comparator-differentiator pair413/423 detects zero-crossings of signal Q (e.g., 512, 516) andgenerates the corresponding triggers (e.g., 522, 526).

Using the signal detected by detector 330 to trigger sampling of theoptical signals arriving at detector 331 provides higher resolution thanconventional sampling based on rising zero-crossings, and also providesuniform wavenumber sampling, even when the frequency of the laser isswept. It should be noted that, because the frequency of the laser isswept, time-synchronous sampling would not provide uniform sampling. Thehigh-resolution, uniform sampling allows discrete Fourier analysis ofthe signals received at detector 331.

The embodiment of FIGS. 3-5 is intended to be exemplary. It iscontemplated that there may be many alternative embodiments whichincorporate variations of the elements described above and still fallwithin the scope of the invention. For instance, while the foregoingembodiment generates eight triggers per period of the reference signal,other embodiments may generate less (e.g., two or four) or more (e.g.,16) triggers per period. Further, while the foregoing embodiment employsa particular arrangement of electronic components (comparators,differentiators, summing unit) to generate the triggers in the triggersignal, it should be understood that many alternative components andarrangements may be used to achieve the desired result (triggers thatprovide uniform wavenumber sampling). Such alternative embodiments arebelieved to be within the scope of the present disclosure. Stillfurther, while the embodiments above are used in connection with asensor array that employs FBGs, the disclosed systems and methods foroptical interrogation may be useful with other types of sensors ordevices, and are not limited to use with FBGs.

Those of skill will further appreciate that the various illustrativelogical blocks, modules, circuits, and algorithm steps described inconnection with the embodiments disclosed herein may be implemented aselectronic hardware, computer software (including firmware,) orcombinations of both. To clearly illustrate this interchangeability ofhardware and software, various illustrative components, blocks, modules,circuits, and steps have been described above generally in terms oftheir functionality. Whether such functionality is implemented ashardware or software depends upon the particular application and designconstraints imposed on the overall system. Similarly, the particularhardware or software components that are chosen to implement thedescribed functionality may be selected to achieve specific designgoals. Those of skill in the art may implement the describedfunctionality in varying ways for each particular application, but suchimplementation decisions should not be interpreted as causing adeparture from the scope of the present invention.

The benefits and advantages which may be provided by the presentinvention have been described above with regard to specific embodiments.These benefits and advantages, and any elements or limitations that maycause them to occur or to become more pronounced are not to be construedas critical, required, or essential features of any or all of theclaims. As used herein, the terms “comprises,” “comprising,” or anyother variations thereof, are intended to be interpreted asnon-exclusively including the elements or limitations which follow thoseterms. Accordingly, a system, method, or other embodiment that comprisesa set of elements is not limited to only those elements, and may includeother elements not expressly listed or inherent to the claimedembodiment.

While the present invention has been described with reference toparticular embodiments, it should be understood that the embodiments areillustrative and that the scope of the invention is not limited to theseembodiments. Many variations, modifications, additions and improvementsto the embodiments described above are possible. It is contemplated thatthese variations, modifications, additions and improvements fall withinthe scope of the invention as detailed within the following claims.

What is claimed is:
 1. An apparatus for triggering uniform wavenumbersampling in an optical frequency domain reflectometry system, theapparatus comprising: an optical coupler configured to receive areference optical signal from a reference interferometer and to provideat least one output optical signal having the same period as thereference optical signal; at least one reference optical detectorconfigured to receive the at least one output optical signal and toconvert the output optical signal into at least one electrical signalhaving the same period as the output optical signal; and a trigger unitconfigured to receive the at least one electrical signal and to generatea trigger signal that contains more than one trigger per period of theat least one electrical signal, wherein the triggers have uniformwavenumber spacing, wherein the optical coupler is configured to splitthe reference optical signal into first and second output opticalsignals, wherein the second output optical signal is 90 degrees out ofphase from the first output optical signal, and wherein the at least onereference optical detector comprises first and second reference opticaldetectors that are configured to convert the first and second outputoptical signals into first and second electrical signals, wherein thesecond electrical signal is 90 degrees out of phase from the firstelectrical signal.
 2. The apparatus of claim 1, wherein the opticalcoupler comprises a 4×4 optical coupler.
 3. The apparatus of claim 1,wherein the trigger unit is configured to generate triggers atzero-crossings of the first and second electrical signals and at timesat which the first and second electrical signals have equal magnitudes.4. The apparatus of claim 3, wherein the trigger unit comprises fourcomparator-differentiator pairs and a summing unit, wherein a firstcomparator-differentiator pair receives the first and second electricalsignals as inputs, a second comparator-differentiator pair receives thefirst electrical signal and an inverse of the second electrical signalas inputs, a third comparator-differentiator pair receives the firstelectrical signal and ground as inputs, and a fourthcomparator-differentiator pair receives the second electrical signal andground as inputs, and wherein an output of eachcomparator-differentiator pair is provided as an input to the summingunit, and wherein an output of the summing unit is the trigger signal.5. The apparatus of claim 1, further comprising the referenceinterferometer, wherein the reference interferometer is configured toreceive a laser light beam and to produce the reference optical signal.6. The apparatus of claim 1, further comprising a laser coupled to thereference interferometer, wherein the laser is configured to produce thelaser light beam and to provide the laser light beam to the referenceinterferometer.
 7. A method for triggering uniform wavenumber samplingin an optical frequency domain reflectometry system, the methodcomprising: providing a reference optical signal having a period thatvaries with a wavenumber of a laser light beam that interrogates asensor array; converting the reference optical signal into at least oneelectrical signal having the same period as the reference opticalsignal; and generating a trigger signal based on the at least oneelectrical signal, wherein the trigger signal contains more than onetrigger per period of the at least one electrical signal, and whereinthe triggers have uniform wavenumber spacing, wherein converting thereference optical signal into the at least one electrical signalcomprises splitting the reference optical signal into first and secondoutput optical signals, wherein the second output optical signal is 90degrees out of phase from the first output optical signal, andconverting the first and second output optical signals into first andsecond electrical signals, wherein the second electrical signal is 90degrees out of phase from the first electrical signal.
 8. The method ofclaim 7, wherein splitting the reference optical signal into the firstand second output optical signals comprises passing the referenceoptical signal through a 4×4 optical coupler.
 9. The method of claim 7,wherein the trigger signal includes triggers at zero-crossings of thefirst and second electrical signals and at times at which the first andsecond electrical signals have equal magnitudes.
 10. The method of claim7, wherein providing the reference optical signal comprises providingthe laser beam to a reference interferometer, wherein the referenceinterferometer produces the reference optical signal.
 11. The method ofclaim 10, further comprising providing the laser light beam to a sensorarray, wherein the sensor array produces an optical sensor signal, andsampling the optical sensor signal in response to occurrences of thetriggers in the trigger signal.
 12. The method of claim 11, furthercomprising sweeping a wavelength of the laser light beam.
 13. An opticalfrequency domain reflectometry system comprising: a laser configured toproduce a laser light beam; a first optical coupler configured toreceive the laser light beam from the laser; a reference interferometerconfigured to receive the laser light beam from the first opticalcoupler and to produce a reference optical signal, wherein the referenceinterferometer is configured to provide the reference optical signal tothe first optical coupler; wherein the first optical coupler isconfigured to receive the reference optical signal and to provide atleast one output optical signal having the same period as the referenceoptical signal; at least one reference optical detector configured toreceive the at least one output optical signal and to convert the outputoptical signal into at least one electrical signal having the sameperiod as the output optical signal; a trigger unit configured toreceive the at least one electrical signal and to generate a triggersignal that contains more than one trigger per period of the at leastone electrical signal, wherein the triggers have uniform wavenumberspacing; a sensor array coupled to the laser and configured to receivethe laser light beam, wherein the sensor array is configured to producean optical sensor signal; and a sensor array optical detector configuredto receive the optical sensor signal, wherein the sensor array opticaldetector is configured to receive the trigger signal and to sample theoptical sensor signal in response to occurrences of the triggers in thetrigger signal, wherein the first optical coupler is configured to splitthe reference optical signal into first and second output opticalsignals, wherein the second output optical signal is 90 degrees out ofphase from the first output optical signal, and wherein the at least oneoptical detector comprises first and second optical detectors that areconfigured to convert the first and second output optical signals intofirst and second electrical signals, wherein the second electricalsignal is 90 degrees out of phase from the first electrical signal. 14.The system of claim 13, wherein the first optical coupler comprises a4×4 optical coupler.
 15. The system of claim 13, wherein the triggerunit is configured to generate triggers at zero-crossings of the firstand second electrical signals and at times at which the first and secondelectrical signals have equal magnitudes.
 16. The system of claim 15,wherein the trigger unit comprises four comparator-differentiator pairsand a summing unit, wherein a first comparator-differentiator pairreceives the first and second electrical signals as inputs, a secondcomparator-differentiator pair receives the first electrical signal andan inverse of the second electrical signal as inputs, a thirdcomparator-differentiator pair receives the first electrical signal andground as inputs, and a fourth comparator-differentiator pair receivesthe second electrical signal and ground as inputs, and wherein an outputof each comparator-differentiator pair is provided as an input to thesumming unit, and wherein an output of the summing unit is the triggersignal.
 17. The system of claim 13, wherein the sensor array comprises asensor array interferometer.
 18. The system of claim 17, wherein thesensor array interferometer comprises a 2×2 optical coupler, wherein afirst port of the 2×2 optical coupler is coupled to the laser to receivethe laser beam, wherein a second port of the 2×2 optical coupler iscoupled to a first optical fiber that terminates at a broadbandreflector, and wherein a third port of the 2×2 optical coupler iscoupled to a second optical fiber that incorporates a plurality ofselectively reflective sensors along the length of the second opticalfiber.
 19. The system of claim 18, wherein the selectively reflectivesensors comprise fiber Bragg gratings.